PROJECT SUMMARY This proposal examines the microbial responses to nutrients by investigating Bacteroides thetaiotaomicron, a prominent gut commensal bacterium that can grow on a large number of carbohydrates that humans and many other microbes cannot utilize. ~18% of the B. thetaiotaomicron's genome is predicted to mediate carbohydrate uptake and breakdown, and the regulation of these processes. This proposal seeks to identify the signals controlling four regulators of carbohydrate utilization, to define their regulated targets, and to establish how these proteins exert their regulatory actions. First, we will investigate the control of and by a master regulator of carbohydrate utilization implicated in gut colonization in several Bacteroides species. This aim also explores the role of a paralog of elongation factor EF-G that is under control of the master regulator, and like the master regulator, it is necessary for gut colonization and carbohydrate utilization. Second, we will examine how glucose, the preferred carbon source in most organisms, silences expression of a transcriptional activator necessary for gut colonization in a diet-dependent manner. And third, we will determine how the essential transcription termination factor Rho, which harbors a prion-like domain-containing element in B. thetaiotaomicron, adopts different forms and controls different sets of genes required for gut colonization and carbohydrate utilization. Our studies will reveal the mechanisms and hierarchies governing carbohydrate utilization, and elucidate the role of both of a prion-like domain in bacterial gene regulation. The proposed investigations may prove paradigmatic because they focus on atypical regulatory proteins. In addition, they may help explain what makes B. thetaiotaomicron a successful gut colonizer, and its abundance in lean healthy individuals.